This summer's hottest — and oddest — cinematic couple is a sarcastic talking raccoon and his alien tree buddy.

Rocket, a furry little dude voiced by Bradley Cooper in Guardians of the Galaxy (in theaters Friday), and Groot, he of the limited vocabulary ("I am Groot") portrayed by Vin Diesel, also happen to be pop culture's newest dynamic duo.

"I think of both those characters as my children," says Guardians director James Gunn. And they're fully realized roles, not just one-dimensional computer-generated parlor tricks. "The fact they're a raccoon and a tree is gravy."

Like the rest of Guardians' misfit crew, Rocket and Groot are cosmic underdogs and cult favorites in Marvel comic books over the years. The movie finds them as wanted bounty hunters who are initially trying to collect some serious dough by capturing fellow outlaw Peter Quill (Chris Pratt).

One chase scene and a run-in with assassin Gamora (Zoe Saldana) later, all four are taken on an adventure that will align them with Drax the Destroyer (Dave Bautista) and oppose the forces of the evil Ronan the Accuser (Lee Pace).

With Groot, a sweet creature yet one who doesn't let anyone mess with his friends, Diesel couldn't have dreamed up a more different character from the tough guys he usually plays. But after signing on, he brought home Guardians concept art and asked his kids, "Which character do you want Daddy to play?"

"They pointed to the tree. I was like: 'OK, so I'm good. The omen is clear," Diesel says. "It is hands-down the most enigmatic, intriguing, mysterious, weird, strange character in the Marvel lot. And that sounded like a fun job."

Because Groot says only three words, and has to communicate different emotions every time he utters them, Diesel worked with Gunn to create a whole new script just for himself.

To further immerse himself in the role, he went into the recording studio wearing jumping stilts. "It may sound crazy. It may sound like it didn't add anything," Diesel says. "I felt like it added a certain commitment and a state of mind for me. He's 7½ feet tall, and you can tell sometimes he maybe wishes he wasn't — there's an awkwardness and an innocence I was able to play with."

Groot's best friend, Rocket, who lacks size but not heavy weaponry, was one of the hardest Guardians to get right in design, says co-producer Jonathan Schwartz.

Rocket also is the crux of the movie from a character standpoint. "If you can believe that Rocket is a real character with real emotions and wants, then the rest of this crazy sci-fi universe becomes easier to buy," he says.

Still, these two unexpected underdogs are on paper a hard sell, and Kevin Feige, Guardians producer and Marvel Studios president, says he has seen many dumbfounded reactions when talking about a movie that takes place in outer space and involves a tree creature and a raccoon.

The usual response: "You're doing what now? Wha huh?!"

But, Feige says, "it was so exciting to us because we knew how cool they were, and if we all do our jobs right, then they would be vying to steal the whole movie. And I think they do."